Nanostructural Tailoring to Induce Flexibility in Thermoelectric Ca3Co4O9 Thin Films

Biplab Paul, Jun Lu, Per Eklund

Due to their inherent rigidity and brittleness, inorganic materials have seen limited use in flexible thermoelectric applications. On the other hand, for high output power density and stability, the use of inorganic materials is required. Here, we demonstrate a concept of fully-inorganic flexible thermoelectric thin films with Ca3Co4O9-on-mica. Ca3Co4O9 is promising not only due to its high Seebeck coefficient and good electrical conductivity but also important due to the abundance, low cost and nontoxicity of its constituent raw materials. We show a promising nanostructural-tailoring approach to induce flexibility in inorganic thin film materials, achieving flexibility in nanostructured Ca3Co4O9 thin films. The films were grown by thermally induced phase transformation from CaO-CoO thin films deposited by rf-magnetron reactive cosputtering from metallic targets of Ca and Co, to final phase of Ca3Co4O9 on mica substrate. The pattern of nanostructural evolution during solid state phase transformation is determined by surface energy and strain energy contributions, while different distributions of CaO and CoO phases in the as-deposited films promote different nanostructuring during phase transformation. Another interesting fact is that the Ca3Co4O9 film is transferable onto arbitrary flexible platform from parent mica substrate by etch free dry transfer. The highest thermoelectric power factor obtained is above 1 ×10(-4)Wm(-1)K(-2) in a wide temperature range, and thus showing low temperature applicability of this class of materials.